1. Field of the Invention
The present invention relates to a process for sintering a powder of cubic system silicon carbide (hereinafter referred to as ".beta.-SiC").
2. Description of the Prior Art
Sintered products of silicon carbide are widely used as abrasion resistant and heat resistant materials by virtue of their superior hardness and high-temperature strength. The silicon carbide is available in two crystal forms, i.e. .alpha.- and .beta.-forms. The present invention is concerned with the process for sintering .beta.-SiC powder.
As a process for sintering .beta.-SiC powder, it has been known to mix the powder with predetermined amounts of boron and carbon, and sinter the mixture in a N.sub.2 gas atmosphere or in an inert gas atmosphere.
In the sintering of .beta.-SiC powder, the difficulty in obtaining a high density product is attributable to the fact that grain growth takes place at the final stage of the sintering and coarse grains having a grain size of e.g. 100 .mu.m or more will thereby be formed in a substantial amount, whereby a high density is hardly attainable. Boron is effective for an increase of the density of the sintered body, but at the same time, it has a function to facilitate the grain growth at the final stage of the sintering. Carbon serves effectively for the removal of SiO.sub.2 (which hinders the sintering) contained as an impurity in the .beta.-SiC powder, but it has been believed that carbon is detrimental if used in excess of the amount required for the deoxidation.
Thus, the amounts of boron and carbon to be mixed with the .beta.-SiC powder used to be restricted to certain specific ranges. For instance, it has been proposed to use from 0.5 to 5.0% by weight of boron and from 1.5 to 5.0% by weight of carbon (Japanese Examined Patent Publication No. 17146/1983), or from 0.3 to 3% by weight of boron and from 0.1 to 1.0% by weight of carbon (Japanese Examined Patent Publication No. 32035/1982). Namely, it has been generally believed that it is necessary to incorporate boron in an amount of at least 0.3% by weight and not to incorporate carbon excessively.
However, these conventional processes still had a difficulty that it was thereby impossible to adequately control the abnormal grain growth and to obtain a sintered body having an adequately high density.
Further, Japanese Examined Patent Publication No. 46996/1980 proposes a process comprising mixing from 0.1 to 5% by weight of each of carbon and boron to a .beta.-SiC powder prepared by a certain specific method and sintering the mixture. According to this process, it is allegedly possible to use boron in an amount as small as 0.1% by weight. However, in all the Examples given in this publication, boron is used in an amount greater than 1.0% by weight, and the use of boron in a smaller amount is not substantiated. Besides, this process has a serious drawback that the .beta.-SiC powder must be prepared by a complicated method.